1. Field of the Invention
The present invention relates to an image forming apparatus, an image formation system and a control method.
2. Description of Related Art
In an image forming apparatus using an electrophotographic scheme, a cleaning device of a blade-cleaning type has been used in which a plate-shaped cleaning blade composed of an elastic body and serving as a device for removing remaining toner such as untransferred toner and transfer residual toner on an image bearing member (for example, photoconductor) is brought into contact with the surface of an image bearing member to remove the remaining toner on the image bearing member, for example.
In recent years, reduction of toner particle size has been demanded from the view point of enhancing the image quality, and for such a purpose, polymerization methods such as the emulsion polymerization method and the suspension polymerization method have been utilized, for example. As the size of the toner particle decreases, however, the attaching force between the toner particle and the image bearing member increases, thus reducing the ease of removal of the remaining toner on the image bearing member. In particular, when so-called polymerization toner produced by a polymerization method is used, the toner particles have a substantially spherical shape, and as a result cleaning defects in which the toner particles roll on the image bearing member and slip through the cleaning blade is easily caused, thus further reducing the ease of removal of the remaining toner on the image bearing member.
In addition, when toner slips through the cleaning blade, the toner becomes the core of toner aggregate formed on the image bearing member, and grain blank (grain noise) is generated on the solid image printing part. To solve such a quality problem as the above-mentioned “grain noise,” today, lubricant is supplied onto the image bearing member to form coating of the lubricant such that cleaning is performed in the state where the attaching force between the toner particle and the image bearing member is suppressed to a low level. Examples of the approach for supplying lubricant on the image bearing member include a lubricant application process in which a brush is brought into contact with lubricant formed in a rod shape to scrape the lubricant and supply the lubricant to the surface of the image bearing member; and a toner adding process in which a toner image is formed with use of toner containing lubricant-external additive (lubricant) to supply the lubricant.
The toner adding process does not require a coating device such as a lubricant rod and a brush and therefore is advantageous in terms of installation space and cost. In the toner adding process, however, the amount of the lubricant in the developing device varies depending on the toner consumption amount, and as a result the amount of the lubricant to be applied onto the image bearing member varies. To be more specific, when an image having a low coverage rate is printed, only the lubricant separated from the toner is preferentially consumed at background parts, and new toner (lubricant) is not supplied to the developing device. Therefore, finally, supply and ejection of the lubricant are balanced and stabilized in the state where the amount of the lubricant in the developing device is reduced. Since only the lubricant is supplied at first, the level of the lubricant on the image bearing member can be maintained at a certain level. However, when the amount of the lubricant in the developing device is reduced, the amount of the lubricant supplied to the surface of the image bearing member is reduced and in turn, the covering rate is reduced. Consequently, the adhering amount between the toner particle and the image bearing member cannot be reduced, and as a result grain noise may be generated.
Conventionally, after an image having a low coverage rate is printed, a toner patch image is formed on the image bearing member in a region outside the image formation region to maintain the amount of the lubricant in the developing device. When the toner patch image is formed, the toner in the developing device is consumed, and the developing device is replenished with new toner. When the toner is supplied, lubricant is also supplied together with the toner, and thus reduction of the amount of the lubricant in the developing device can be suppressed. As a result, reduction of the amount of the lubricant on the image bearing member can be suppressed. Conversely, after an image having a high coverage rate is printed, the amount of the lubricant on the image bearing member increases as lubricant is supplied into the developing device together with the toner. When the amount of the lubricant on the image bearing member is excessively increased, the cleaning blade makes close contact with the image bearing member with the lubricant therebetween, and consequently problems such as turn-up of the blade are caused. That is, the amount of the lubricant on the image bearing member has its proper range, and therefore the amount of the lubricant is required to be stabilized.
It is known that the toner patch image has an effect of abrasion of the lubricant applied on the image bearing member (abrasion effect), and an effect of supplying lubricant together with toner onto the image bearing member (supply effect). The amount of the abrasion depends on the amount of scraping and removing the lubricant applied on the surface of the image bearing member. The supplying amount depends on the amount of toner supplied onto the image bearing member, and the amount of the lubricant removed from the toner. Which of the abrasion effect and the supply effect is obtained depends on the balance between the abrasion amount and the supplying amount. In view of this, toner patch image is used paying attention to one of the effects to be obtained in accordance with the system and condition.
Japanese Patent Application Laid-Open No. 2011-7831 discloses a technique for improving the cleaning performance by maintaining the amount of the lubricant on the surface of a photoconductor in an image forming apparatus of a contact transfer type without requiring improvement of the cleaning performance of the transfer section (transfer belt). In the technique disclosed in Japanese Patent Application Laid-Open No. 2011-7831, a toner image based on a toner forcible-ejection patch is formed in a non-image region on the photoconductor with use of toner mixed with lubricant for lubricating the surface of the photoconductor, and the polarity of the transfer belt is changed to a polarity same as the polarity of the toner having at the time when the non-image region of the photoconductor makes contact with a transfer belt, whereby the toner image is prevented from being transferred to the transfer belt.
However, since the effect obtained with the toner patch image depends on the balance between the abrasion effect and the supply effect, the desired effect may not always be obtained. When the desired effect cannot be obtained, it has been difficult to suppress the variation of the amount of the lubricant to be applied to the image bearing member to maintain the lubricant within a proper range. For example, after a large number of high-coverage rate images are formed, the abrasion effect is desirably obtained through formation of the toner patch image; however, the supply effect may instead be exhibited and consequently turn-up of the blade may be caused due to an excessive amount of the lubricant. In addition, after a large number of low-coverage rate images are formed, the supply effect is desirably obtained through formation of the toner patch image; however, the abrasion effect may instead be exhibited, and consequently grain unevenness may be caused due to an insufficient amount of the lubricant. As described, with the conventional techniques, the effects obtained with the toner patch image could not have been appropriately controlled.
Japanese Patent Application Laid-Open No. 2011-7831 discloses a technique for maintaining the amount of lubricant on the surface of the image bearing member. However, Japanese Patent Application Laid-Open No. 2011-7831 does not states that which of the abrasion effect and the supply effect is obtained with formation of the toner patch image, which is prerequisite technique to solve the above-mentioned problems, and therefore, the above-mentioned problems cannot be solved with the technique disclosed in Japanese Patent Application Laid-Open No. 2011-7831.